When not so accustom to downloading torrent files from public sources, it can be very difficult to distinguish differences between a safe one and a harmful one. After downloading, possibly, hundreds of torrents you will definately know the difference purely by experience. But, for now, you can read these pointers.
First of all, and possibly most important of all, is checking the source. http://www.thepiratebay.org/ is a great source, but often goes temporarily offline because of the size of the publicity it gets. Websites like this that allow comments are very helpful. If you have several pages of positive comments, then you can be sure that the respective torrent file is safe for you to download. However there are a few torrent uploaders that have people they know to post positive comments, when in fact the torrent is potentially harmful. Be cautious of that.
Websites with no advertisements are too good to be true and most likely cannot be trusted, because torrent websites have to make money in order to support the website. Pages with no adverts will probably tell you that this website is fraudulent and obviously makes its money through scams.
Next you should study the contents of the torrent file. If it is just an .exe file by itself, then more than likely it is a threat in every case. Apart from if you are downloading a crack or a keygen (key generator) for a game or other program. Game cracks are usually fine, just compare the file size to the original .exe. If they differ in size too much then it should not be trusted. As for keygens, I do not usually approve of downloading them, as they have a high risk. If you need a key, you can usually find a list somewhere in the comments section of the torrent page or on the internet.
Another similar case to this is .zip files. When you open the torrent file it will only show the .zip file and not the actual contents. However, .zip files are always safe to open, particularly when using software like WinZip or WinRar. So you can examine the contents of the .zip safely by opening it (not extracting it). The disadvantage is that you must decide whether to download it or not before you know if it is safe.
Here is of list of what you would expect in a torrent file:
Music: .mp3, .wav, .m4a
Video: .mp4, .avi (There are several various movie formats, but these are the main two.)
PC Games: .iso, .nrg, .mdf (Normally virtual disk files such as these, I would not trust any other form.)
PSX, Xbox, PS2 Games etc: .iso, .mdf, .img, .bin, .ccd
Software: Install or Setup.exe, .iso
Cracks and Keygens: .exe
Ensure you have a good anti-virus program installed, some free ones are SUPERAntiSpyware and AVG. I would not recommend Norton Anti-Virus because it eats CPU for breakfast. There is no sense in scanning the torrent for threats because the torrent merely carries the information for your computer to find and download the files.
Before you do anything else with the newly downloaded files; scan the whole folder. With SUPERAntiSpyware you can right-click the directory and click the “Scan with SUPERAntiSpyware” option. Depending on the size the scanning process may take a few moments. If you are unfortunate enough to have downloaded a virus then all other files that came with it must be deleted and removed from the Recycle Bin immediately. Normally your anti-virus program will move it to a Virus Vault. This is better than deleting it because you can keep it with no risk and decide later if you want to keep it or not. If it a keygen that is causing the problem I would rid of it and possibly keep the rest. The keygen is not vital like a plugin or the program itself.
If you are really that worried about your computer being attacked by a virus you can set a restore point. Click Start > All Programs > Accessories > System Tools > System Restore. Select the option to “Create a restore point” and then go through the wizard-guide to set a restore point. Once you have done this you can download your files and if your computer does get a virus you can go back to System Restore. In there select the option to “Restore my computer to an earlier time”. Select the most recent restore point (assumingly the one you created) and your computer will begin restoring itself to before you downloaded the files. It will then reboot and hopefully it should be back to normal. Then you can go to where you downloaded the torrent and post a thankful comment to the uploader.
Lastly have a look at how many peers and seeds there are. For torrents with viruses there should be a very low number of seeds, practically none. This is because people would definitely not seed virus, and additionally they would have already deleted the files because obviously it is a threat to their computer.